2 - Progression of severe accidents in water cooled reactors

Abstract

In this chapter, we will discuss the progression of severe accidents in light water reactors (LWRs) and heavy water reactors (HWRs). A severe accident is said to occur only when the accident conditions are more severe than a design basis accident and involve significant core degradation in terms of melting of core. A severe accident can be initiated by reactivity insertion accidents and core uncovery accidents. In LWRs, the core uncovery accidents can occur due to unmitigated loss of coolant accident or an extended station blackout. In HWRs, if the moderator cooling system is unavailable either in an unmitigated loss of coolant accident or an extended station blackout, it will lead to a severe accident. During the progression of a severe accident, many complex phenomena involving core degradation and relocation, melting of core debris, interactions with structures and fluids involving difficult thermodynamics, complex material chemistry, etc. occur. Each of the phenomena is a function of temperature of fuel, clad, and structural material including that of the coolant, which can occur in a time period of few seconds to few hours. The consequence of a severe accident is very high; hence, severe-accident management strategies have been adopted for operating water-cooled reactors as well as new reactors. The most important aspect is to cool and stabilize the molten core either inside the vessel of LWRs or inside the calandria of pressurized hard water reactor s, which is known as “in vessel retention.” Some of the large-sized LWRs use ex-vessel dedicated core catchers to cool and terminate severe accident progression. Their core catchers use a special sacrificial material to absorb the heat from a molten corium without causing any recriticality and also avoid the formation of combustible gases such as hydrogen during the water flooding. Steam explosion is another potential hazard during severe accident progression when a molten corium interacts with subcooled water. The phenomena of steam explosion are complex to model from the first principle similar to other phenomena of severe accident. To reduce the uncertainties in quantification of the phenomenology, experiments are the major strategies that supplement necessary correlations to the computational models.